1. Field of the Invention
The present invention relates in general to a voltage regulator, and more particularly to a voltage regulator capable of decreasing a leakage current from output transistors when a load is light.
2. Description of the Related Art
As shown in FIG. 3, a conventional voltage regulator includes a voltage regulator control circuit having a reference voltage circuit 2, bleeder resistors 6 and 7 for voltage-dividing an output voltage Vout appearing at an output terminal 5 of the voltage regulator, and an error amplifier 3 for amplifying a voltage difference between a reference voltage Vref from the reference voltage circuit 2 and a voltage Va appearing at a node between the bleeder resistors 6 and 7, and an output transistor 4. The voltage regulator operates at a power supply voltage VDD supplied from a voltage source 1.
When an output voltage of the error amplifier 3 is assigned Verr, if the voltage Va is larger than the reference voltage Vref, the output voltage Verr becomes higher, while if the voltage Va is smaller than the reference voltage Vref, the output voltage Verr becomes lower. When the output voltage Verr of the error amplifier 3 becomes higher, the output transistor 4 becomes larger with its ON resistance to serve to decrease the output voltage Vout. On the other hand, when the output voltage Verr of the error amplifier 3 becomes lower, the output transistor 4 becomes smaller with its ON resistance to serve to increase the output voltage Vout. The voltage regulator control circuit holds the output voltage Vout constant by those mechanisms.
Since resistance values of the bleeder resistors 6 and 7 are generally set to very large values, the voltage regulator controls the output transistor 4 so that the ON resistance of the output transistor 4 becomes very large when a load is light (e.g., refer to JP 07-74976 B (FIG. 2)).
However, if the output transistor 4 is so controlled that its ON resistance becomes very large when the load is light, there is encountered a problem in that an influence of a leakage current from the output transistor 4 becomes larger and hence the output voltage Vout cannot be held constant. In general, when a channel width and a channel length of a transistor are assigned W and L, respectively, a W/L value is reduced to allow a leakage current from the transistor to be reduced. In this case, however, an amount of current which can be caused to flow when a load is heavy becomes lesser accordingly.